The present invention pertains to battery-powered devices, and, more particularly, pertains to an apparatus and method for obtaining incremental variable voltage from switchable configurations of banks of battery cells.
It is well known that battery-powered devices often require variable voltage output from a power supply in order to perform normal functions. An electric vehicle drive is a good example of this kind of device. Typically, the fill voltage of the battery bank is switched on and off at a rapid rate to turn the constant voltage of the battery bank into a lower effective voltage to the load. Voltage is made lower by leaving the switches off for longer periods of time; and, conversely, voltage is made higher by leaving the switches on for longer periods of time. This method of voltage control is sometimes called Pulse Width Modulation (PWM) or Pulse Frequency Modulation (PFM). As will be understood by those skilled in the art, for the electric vehicle, voltage is low for slow speeds and higher for faster speeds.
These common methods of voltage control have several limitations that are known in the art. First, pulsing the voltage and, consequently, the power, on-and-off requires a relatively complex circuit to turn the power-controlling components on-and-off at a rapid rate. Second, the pulsed output requires expensive filtering components to remove high frequency harmonics from the power being delivered to the load. Next, the control range is only 4 to 1 for many high power applications. This limitation occurs, of course, because the current requirements for loads such as DC motors do not decrease with voltage. As the PWM or PFM method reduces the time that the power controlling device is on to reduce the voltage, current--that can only flow when the device is on--is squeezed into narrower time pulses. To keep the average current to the motor constant, the current flow during the pulse must rise. For example, if the voltage output were reduced to one half of maximum by leaving the power controlling device off for half of the time, then current during the on-time of the device must be twice as high. At one quarter voltage output, current must be four times as high. Solid state devices, in particular, have definite maximum current limits. Increasing the controlling range of a PWM or PFM system increases the maximum current flow and the cost of the components. Furthermore, if regenerative braking is used, another power circuit is needed to control the current flow back to the batteries.
There have been several improvements in the art to extend the reach of battery power. For example, Field et al., in U.S. Pat. No. 4,788,480, teach an apparatus for controlling the output of battery-driven motors, wherein either of two available voltages is obtained from a portion of the battery supply while simultaneously avoiding unequal battery-drainage. In U.S. Pat. No. 5,710,504, Pascual et al. disclose a method for equalizing recharging of a bank of batteries configured in a series relationship. Having a plurality of switches for controlling current flow, charge is shifted between two adjacent batteries so that the action of repeated back-and-forth capacitor-switching equalizes the voltages of the batteries in the battery bank. Similarly, in U.S. Pat. No. 5,712,553, Hallberg teaches an apparatus including a transposition circuit that uses switches for selectively changing the order of series-connected batteries, for equalizing battery consumption in a power supply that provides at least two uniform and different voltage outputs.
Yang attempts to improve the art in his disclosures in U.S. Pat. Nos. 5,506,456; 5,461,264; 5,483,643; and 5,514,480. More particularly, in his '264 and '456 patents, Yang teaches apparatus that interconnects a bank of batteries in a series and parallel relationship using a plurality of switches, for equally draining of these batteries. It should be dear to those skilled in the art that the battery-switching methodology used in this apparatus effectuates large voltage level jumps at the upper voltage range. To deal with this situation, an additional modulation circuit is included to vary the voltage across these large jumps, which, of course, significantly increases the complexity and associated cost thereof. While requiring many components, this apparatus keeps all of the batteries in the circuit so that equal draining inherently occurs under all load conditions. In the '916 patent, he discloses an apparatus for reducing sparking by combining solid state switching with electromechanical switches and, in the '643 patent, Yang teaches a transforming circuit for converting DC to variable voltage AC. In the '433 patent, he discloses a voltage control circuit for providing incremental voltage control that, unfortunately, engenders considerable power loss attributable to a large number of prerequisite components therein.
It is well known that battery banks are composed of individual low voltage cells usually connected in series. When batteries are connected in series, the voltages of all the cells are added together to produce the output voltage of the total battery bank. This output is normally applied to a PWM or PFC controller to vary the voltage. Heretofore unknown in the art is a manner or means for varying the output voltage from a battery bank by varying the number of battery cells placed in series. An advantage of this approach would be that the many incremental voltages inherently available inside the battery bank would be available for use, while avoiding the complex circuitry need to produce the incremental voltages otherwise, i.e., artificially. Of course, it is common practice in the art to use battery banks having a large plurality of switches to provide varying power, but such arrangements are also complex and inefficient.
Thus, it is dear that there is presently no device or procedure that enables incremental voltage control to be simply attained from a switchable bank of low voltage battery cells. Accordingly, these limitations and disadvantages of the prior art are overcome with the present invention, and improved means and techniques are provided that are useful for effectively and inexpensively controlling the voltage produced by a bank of battery cells.